This invention relates to medical ultrasonic imaging, and in particular to a new beamforming method and apparatus that require only a few transmit/receive events to form an entire frame of an image.
Commercially available medical ultrasonic imaging systems use a large number of transmit/receive events for each frame of the image. Each transmit event steers a pulsed beam of ultrasonic energy along a particular scan line and focuses this energy to a particular focus depth. After each transmit event, echoes are received, amplified and digitized. The receive beamformer generates a line of the image by dynamically focusing and apodizing the receive signals along a scan line. To increase transmit depth of field, some systems use multiple transmit/receive events per scan line, each transmit event focused at a different depth. To increase frame rate, some systems use multiple beamformers that can generate multiple lines of an image per transmit/receive event. With all these conventional approaches, each frame of the image is constructed from a large number of scan lines (typically 50 to 250, depending on the lateral resolution and total scan extent) and therefore from a large number of transmit/receive events. The frame rate is ultimately limited by the total number of transmit/receive events, because each transmit/receive event takes a finite amount of time determined by the speed of sound, maximum depth of interest and system overhead. This limitation is particularly acute for three-dimensional imaging. Examples of commercially successful ultrasonic imaging systems of this type are described in the following U.S. patents, all assigned to the assignee of the present invention: U.S. Pat. Nos. 4,550,607; 4,699,009; 5,148,810; 5,235,986; 5,573,001; 5,608,690; 5,623,928; 5,675,554; 5,685,308.
Various unconventional techniques have been proposed to increase transmit depth of field without having to use multiple transmit/receive events per scan line. In one of these proposals, simultaneously fired multiple plane waves with different steering angles were used to form limited diffraction transmit beams called Sinc Waves [1]. Sinc Waves were demonstrated to maintain the lateral field response over a greater depth of field than even a Gaussian apodized transmitter. However, this method required element to element variation of the transmit pulse waveform.
Later, another method was reported which eliminated this disadvantage and combined the synthesized transmit Sinc Wave with receive dynamic focusing [2]. In this method plane waves with different steering angles were fired sequentially and the image was synthesized using all transmit/receive events. This method, on the other hand, has the disadvantage of a slow data acquisition rate due to many transmit/receive events (in the example given 41 successively fired plane waves were used). If the object moves and/or the user scans the object during these transmit/receive events, serious motion artifacts result. It was also reported that if the number of transmit/receive events, i.e., the number of plane waves, were reduced, then the side lobes were adversely affected.
Other methods have also been devised to form limited diffraction beams. One of the limited diffraction beam types, called X Waves, was used to develop a very high frame rate image construction technique called the Fourier method [3]. In this technique the array transducer is excited to produce a plane wave. From the received and recorded echo, the limited diffraction response is produced by a bank of step-wise sine and cosine apodizations. The multi-dimensional spectrum of the object is derived from the temporal Fourier transform of the apodized signal. The image is then constructed through the Inverse Fourier Transform of the spectrum. Since a single transmit/receive event is used and the Fourier and inverse Fourier transforms can be computed at high speed with FFT processors, this technique was reported to have the potential to achieve very high frame rates with simple hardware. The images formed with this technique were reported to be detail and contrast resolution equivalent to images generated by a conventional dynamic receive beamformer operating with a pulsed plane wave excitation on transmit. This technique therefore provides only half the maximum achievable lateral spatial bandwidth.
Another technique that was proposed to form real-time 3-D ultrasound images used Sparse Synthetic Aperture Beamforming [4]. Each frame of image is synthesized from a few sets of echoes, where each set of echoes is in response to a spherical wave insonification of the object. This method suffers from low SNR because only a few elements are used for each transmit firing and the amplitude of diverging spherical waves is inversely proportional to the propagation distance. To make up for the SNR deficit, it was proposed to increase the number of elements per transmit firing and to use many firings per frame.